Collimated Outflows in Planetary Nebulae

Our understanding of the formation of planetary nebulae (PNe) has been profoundly influenced in recent years by the detection of high-velocity, collimated outflows in these objects. Outflows reaching expansion velocities of several hundred km s and evidences of episodic, multiple ejection events since the proto-PNe stage, have radically modified our previous concepts of the evolution of post-AGB objects. Hydrodynamic models alone seem now incapable of reproducing many of the attendant dynamical phenomena observed in PNe. MHD models and binary nuclei are thus playing an increasingly important role in interpreting the diversity of collimated outflows in PNe. This paper presents an overview of these phenomena and discusses some of the current challenges and trends in the field.